There are two things that have been bugging me lately..
1) The Match.com adds that say that 'couples that use match.com are 30% more likely to find a partner then people who don't'. They don't let you know if it's [people who use match.com and are looking for a partner] vs [people who don't use match.com and are looking for a partner], or if it is, as I suspect.. [people who use match.com] vs [everyone else, both looking and not looking for a partner].
2) There is a life insurance add (can't remember the company) that has people put a circle on a billboard in a park asking them what the age of the oldest person they know is. of COURSE there will be a lot more circles on the 'upper' ages! you have multiple people who could know the same much older person, inflating the scale upward.